The present invention relates to a process for producing shrinkage-matched composites which comprise at least one ceramic component and the composites obtainable by this process.
The production of materials-based components frequently includes the problem of combining various materials (e.g. ceramic and metal or ceramic I and ceramic II) with one another to form one unit. If such composites are produced by powder-metallurgical methods, there is frequently the additional requirement of first pairing unlike materials in the unfired state in order to then sinter them together in a co-firing to produce the actual product.
However, the production of unlike material pairings via sintering processes presents the fundamental problem that each component has specific material properties such as modulus of elasticity, coefficient of thermal expansion and material transport properties (sinter activity). The differences in the sinter activity lead to different shrinkages during the sintering process, so that distortion or complete destruction of the component can occur already during the densification phase. Furthermore, intact components frequently have very high residual stresses which are attributable, on the one hand, to the different shrinkages on sintering and, on the other hand, to the differences in the coefficients of thermal expansion of the materials involved. There have therefore been many attempts to master the problems of the different shrinkages on sintering and different coefficients of thermal expansion by materials-related measures. Among the most widely employed techniques is the use of low-temperature-sintering glass phases by means of which, in combination with appropriate starting powders, sintering temperature and shrinkage on sintering are adjusted. However, for many applications glass phases cannot be used since they adversely affect the material properties of one or more components or the required properties are not achieved at all in the system. In these cases, attempts are made to realize the different material pairings by means of the grain size dependence of the sinter activity of powders. This method of matching the shrinkage on sintering can be employed for many material combinations, but it is associated with considerable technical complication and financial cost. In complicated processes, the grain size distributions of the components have to be matched to one another or powders of appropriate fineness have to be produced first. All techniques have in common the fact that not only selected raw materials but also organic additives are required. These additives, on the one hand, are to disperse the powders homogeneously and in a deagglomerated state in a dispersion medium. On the other hand, they assume the function of processing aids by means of which the rheological properties or the processability of the ceramic compositions are matched to the requirements of the respective shaping process. Depending on the shaping process, the proportion of organics can be up to 50% by volume and has to be removed before sintering, sometimes by complicated and time-consuming processes. This step becomes particularly difficult when different materials such as ceramic/metal are joined to form one part, since the removal of the organic processing aids then has to be additionally carried out under inert conditions. In order to overcome these difficulties and limitations, it would be extremely worthwhile to develop techniques, processes or materials which permit very substantial matching of the shrinkages on sintering when using unlike materials and at the same time make do without addition of organic processing aids or make do with greatly reduced proportions of these. In the ideal case, the function of the organic processing aids should be taken over by inorganic materials which are converted into the ceramic material during sintering.
It is therefore an object of the present invention to provide a process for producing composites which comprise at least one ceramic component, which process enables the shrinkage behaviour of this or these ceramic component(s) on sintering to be matched to that of the other ceramic and/or non-ceramic components.
It has surprisingly been found that this object is achieved by using nanosize ceramic powder as starting material (or a constituent thereof) for the ceramic component(s) whose shrinkage behaviour on sintering is to be matched to the other component(s). This process provides completely new possibilities in pairing materials and optionally in densifying them in a co-firing.
The present invention accordingly provides a process for producing composites which comprise at least one shrinkage-matched ceramic component, which process is characterized in that the starting material for the ceramic component(s) whose shrinkage behaviour on sintering is to be matched to the remaining component(s) is selected such that the ceramic-forming constituent of the same consists essentially of:
(a) at least one ceramic powder (i) comprising particles having a size of up to 500 nm;
(b) at least one ceramic powder (i) as defined in (a) in admixture with at least one powder (ii) comprising at least one sintering-inhibiting substance having a particle size equal to or smaller than that of the powder (i); or
(c) at least one ceramic powder (i) as defined in (a) in admixture with at least one ceramic powder (iii) having a particle size above that of the powder (i) used and up to 500 xcexcm.
The ceramic powder (i) preferably comprises particles having a size of up to 300 nm and in particular up to 200 nm. Although there is no critical lower limit for the particle size, for reasons of being able to make the powder it is usually about 1 nm. Furthermore, the ceramic powder (i), like the other ceramic powders used, can be used in pretreated form; this pretreatment can include, in particular, a surface modification of the powder particles with short-chain (preferably bifunctional) organic or organometallic compounds, as described, for example, in DE-A-4212633. The purpose of this pretreatment can be, for example, to adjust the rheology of the mix and/or (particularly in the case of nanosize powders) the solids content.
The particles of the ceramic powders used in the present invention can have various shapes, for example spherical, tabular, fibre-shaped, etc. The term particle size as used herein refers in each case to the longest dimension of these particles, which corresponds, for example, to the diameter in the case of spherical particles. Furthermore, for example, agglomerates can first be produced from these powders and then be subjected to thermal post-treatment in order to adjust the sinter activity.
The ceramic materials used in the present invention are preferably derived from metal (mixed) oxides and carbides, nitrides, borides, silicides and carbonitrides of metals and nonmetals. Examples are (optionally hydrated) Al2O3, partially and fully stabilized ZrO2, mullite, cordierite, perovskites, spinels, e.g. BaTiO3, PZT, PLZT, etc., and also SiC, Si3N4, B4C, BN, MoSi2, TiB2, TiN, TiC and Ti(C,N). Of course, it is also possible to use mixtures of oxides or mixtures of non-oxides and mixtures of oxides and non-oxides. Particularly preferred ceramic starting materials are (xcex1- or xcex3-)Al2O3 and ZrO2 (in unstabilized, partially stabilized or fully stabilized form).
The above alternative (b) of the process of the invention is of particular interest when composites are to be produced from a (ceramic) powder which sinters relatively sluggishly and a powder (i) (e.g. in the production of filters). In this case, the shrinkage behaviour of the (very sinter active) powder (i) has to be matched to that of the coarser powder. Sintering-inhibiting secondary phases in the form of the powder (ii) are used for this purpose. The particular materials used as powder (ii) depend on the nature of the powder (i). For xcex1-Al2O3 as powder (i), the powder (ii) used can be, for example, SiC, mullite or ZrO2. In this way, it is possible, for example, to sinter a thin Al2O3/SiC layer onto a porous Al2O3 substrate, i.e. coarse Al2O3 having a very low sinter activity, in a co-firing and to produce defined pores whose size depends on the size of the particles in the powder (i) in the Al2O3/SiC layer. This is not possible using known techniques.
The powder (ii) has to have a particle size less than or equal to that of the powder (i) used in combination. Furthermore, the powder (ii) is preferably a ceramic powder. The weight ratio of powder (i) to powder (ii) (and in the case of the alternative (c), to powder (iii)) is not critical and depends on the circumstances of the individual case.
The powder (iii) used according to the invention can be, for example, a commercially available sinterable powder or an agglomerated (granular) material derived therefrom. It can, however, also be an agglomerated material derived from a powder (i).
The one or possibly more remaining component(s) of the composite to be produced according to the invention can be selected from among various materials which can be processed in combination with a ceramic material to give a useable composite and can withstand a sintering process. Further components which are preferred according to the invention are selected from among ceramic and/or metallic materials and/or glass. Particularly preferably, at least one further component consists of ceramic material, preferably a material whose starting components are selected from the above ceramic powder (iii), e.g. a ceramic powder (iv), in which primary particles having a size of up to 500 nm, preferably up to 300 nm, are present in the form of agglomerates. Furthermore, it is of course also possible for the composite to be produced according to the invention to include a plurality of components derived from the alternatives (a), (b) and (c).
According to the invention, it is particularly preferred that the chemical composition of all powders (i), (iii) and (iv) used for producing a composite is identical.
The above ceramic powders (iv) having agglomerated primary particles can be produced from the corresponding powders (i) by conventional methods. Concrete examples of such methods are indicated below in the Examples.
The composites comprising at least one shrinkage-matched ceramic component obtainable according to the invention can be produced by means of any process which is suitable for producing a particular composite of the envisaged type. Examples of such processes are pressing, electrophoresis, gel casting, rolling, slip casting, injection moulding, tape casting, pressure casting and lamination. Such processes are well known to those skilled in the art. The processing of the ceramic starting material to give the finished ceramic is also carried out in a customary way. Examples of processing methods are likewise indicated in the Examples.
The process of the invention makes it possible to obtain, for example, (multi-layer) gradient materials in which a porous support is provided with a dense layer or a support having coarse porosity is provided with a layer having fine porosity, it being possible to adjust the pore size and porosity also by means of the sintering process. Of course, multilayer structures in which each layer is dense can also be produced according to the invention. The same applies to multilayer structures in which, for example, porous and dense layers alternate, it being possible for the layers to consist of identical or different materials. The invention enables, for example, composites (consisting exclusively of ceramic material) in the green state to be sintered in a co-firing or green and sintered parts to be joined by sintering.
Composites which are particularly preferred according to the invention are multilayer (e.g. two-layer) structures composed of ceramic sheets, in particular filters or filter components. Another preferred embodiment comprises composites of ceramic sheets and metals.
The process of the invention has various advantages. In particular, alternative (a) enables, in comparison with processes which use powders having significantly larger particle sizes, significantly higher sinter activities (i.e. significantly lower sintering temperatures) to be achieved. At the same time, completely new material combinations, e.g. in the field of ceramic/metal, which can be densified by means of a co-firing are also possible. A further important aspect is that advantages are obtained in the sequential build-up of, for example, multilayer structures. Thus, new material combinations and structural designs on sintered substrates can be realized. Likewise, it is possible to seal or even out surface flaws by means of thin layers of ceramic.
The advantages of the alternative (b) have already been briefly discussed above.
According to the alternative (c), variation of the proportions by mass of powder (i) and powder (iii) enables the shrinkage behaviour to be conveniently matched. Of particular interest are the opportunities given by the use of a powder (iii) which has been produced from the powder (i) by agglomeration (powder (iv)) together with a pure powder (i). This may be briefly illustrated for a two-layer structure:
A sheet A is first produced from the powders (i) and (iv). A second sheet B produced from the powder (i) is cast or laminated onto the sheet A. In a co-firing, the two sheets can be sintered to one another to give a stress-free composite. Depending on the composition of sheet A and the sintering conditions employed, dense materials, materials having a dense layer (sheet B) and porous support (sheet A) or porous layer (sheet B) and porous support (sheet A) can be obtained. This is not possible using known techniques.
The present invention is illustrated below by means of non-limiting examples.